A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Electrostatic clamps may be used in lithographic apparatuses operating at certain wavelengths, e.g., EUV, since at these wavelengths, certain regions of the lithographic apparatus operates under vacuum conditions. An electrostatic clamp may be provided to electrostatically clamp an object, such as a mask or a substrate (wafer) to an object support, such as a mask table or a wafer table, respectively.
The voltage that is applied to an electrostatic clamp may be considerable. For example, the voltage may be of the order of kilovolts. An insulating barrier is conventionally located over an electrode of an electrostatic clamp, the insulating barrier acting to insulate an object such as a substrate from the voltage applied to the electrode. The insulating barrier may be thin, in order to minimise voltage drop across the insulator. However, the thinness of the insulating barrier may make it susceptible to cracking. If cracking occurs then a discharge between the electrode and the substrate may occur, which may cause damage to the electrostatic clamp and to the substrate. In such circumstances it may be necessary to replace the electrostatic clamp and to discard the substrate.
It is desirable to provide, for example, an improved electrostatic clamp that obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.